I've looked for this but I haven't found it. If there's already a thread that talks about this, then please point it out.

I half want to know this for a fan fiction I'm writing but mostly out of curiosity. How long do ponies live in the MLP universe?

I tried using Apple Family Reunion as a reference. But there are so many unknowns, like if 100 moons mean lunar cycles or if there's some other meaning behind the word. Also, even if it does mean lunar cycles, there's never been much reference to how the moon works beyond Luna does it. So maybe there's only four lunar cycles in a year, or even three in a month. Although, considering the last Apple family reunion was when Apple Jack was a baby, I doubt it's that often. Even then, even if I did know how often the reunions happen, or even exactly how many family reunions have happened, at best I would be able to get a general idea of how old Granny Smith is, who is still alive (at around age 50 if their lunar cycles are the same as ours, which means they do not age the same way horses and ponies age in our universe).

So I've hit a dead end with that. So I was wondering if anyone else has any theories about the lifespan of the MLP ponies.

Also, I wonder if there's any age difference between unicorns, pegasus and Earth Ponies. I know there's a major age difference between the alicorns and the other ponies, but I wonder what kind of age differences there are between the different types of ponies.

Good question. My guesses are, in human years.
Alicorns: 3000 years (Godlike but not immortal)
Unicorns: 90-180 (Many become sage/wizard types and I'd imagine they live at least to Gandalf/Dumbledore years)
Earth ponies / pegasi: 60-85
Dragons: About a millennia
Changelings: A year tops
Crystal Ponies: 100-150 years (Have a hunch that the minerals in their bodies deteriorate slower than fleshy ponies).

Well, I don't know about the others, but I know that Apple Jack is about 100 moons old (give or take). What that means in human years I have no idea. And I know Apple Jack is the oldest of them all, so the others are at younger than a 100 moons.

But over all, I guess the general consensus is that the ponies in the MLP universe have a similar lifespan to humans.

Crystal Ponies: 100-150 years (Have a hunch that the minerals in their bodies deteriorate slower than fleshy ponies).

I like this theory.

Although, I know another way to figure out how old at least Granny Smith is, is to know how old Ponyville is, since she was one of the ponies who helped found the town, and that she was fairly young (although old enough to have discovered her cutie mark) when they started building. I guess we'll have an answer if there's ever an episode where ponies celebrate the founding of their town.

No, not at all. Celestia and Luna are immortal for reasons unknown, but regular ponies do get old and die. In fact, in the Hearts and Hooves Day episode, while the Cutie Mark Crusaders are singing their song about finding a stallion for Cheerilee, one of them actually interrupts a funeral to say that the pastor is too old. In addition, Granny Smith is very old, as well, and it's explained that she played an important role in the founding of Ponyville. Of course, we don't know how old Ponyville is because it's never explicitly stated.

I'd say ponies have around the same age span as humans do. According to Google, 1 horse year is around 6 1/2 human years.
So if ponies can live for 70+ years in the show, that would mean they'd be around 450 human years old.

So in that regard, Fluttershy says she's a year older than Pinkie pie. Does that mean 6 1/2 years older, or just one?

This is all kind of a guess though. For all we know, Granny Smith could only be seven months old, but in Equestria ponies age really slowly.

I think it depends on the timeline of their age. Luna & Celestia are immortal, and they will live on for a long time until they eventually die. Ponies don't usually live on forever, so they will eventually die when they do pass on.

Its already proven that both age and death do occur in Equestria but its anybody's guess as to how long the average life span is for a pony

I myself don't follow the actual equine vs human age variences so for myself I say it may be somewhere in between. (But you will never convince me that any of the main 6 are less then 18 in human years) I also don't buy into the argument that Celestia and Luna are immortal. I do feel that their longevity had more to deal with their connection to the Elements of Harmony and now that that connection has been severed Both Celestia and Luna are feeling time catching up with them (I do believe Celestia is loosing her powers)

But again this is all personal speculation because none of this has been addressed cannon-wise

Real horses usually live to 25 or 30, smaller ponies can often live for a little longer although they usually don't age past 40. However since the ponies are pretty much humans in the show I can be fairly sure they live a little past 40 years old. To keep a balance between reality and fiction I would have to guess that the average lifespan for a pony in the show is about 60-65 years. Based on this Fillies like the CMC are probably 5-6 (10-12 in human years), the mane 6 are around 20-25 (25-30 HY) Granny Smith (who I'm assuming is the oldest one on the show) would be around 60 (90 HY).

I don't like to believe the alicorns are immortal, but seeing as celestia is at least 1000 and still in what seems to be her prime I say it's fair to say that they live to be very old, older than Granny Smith even!

In context of the show, it can be assumed that a pony's natural lifespan is at least the same as that of a human. It's fairly clear that the ponies in MLP are not the same as ponies in real life, which typically live around 20 to 30 years. There is very little to suggest that MLPonies age like real ponies do.

We know that more than a year has passed in the series, given that we have seen winter at least twice (first in "Winter Wrap Up," and again in "Hearth's Warming Eve"). In this timeframe, young adult horses would have aged between four to five human years. There is very little indication of this, especially with regards to the Cutie Mark Crusaders and their peers. A real foal would have grown up; the CMC have barely changed at all.

There are other factors that help discredit the idea that MLPonies age at the same rate as real horses. If MLPonies aged as quickly as their real life counterparts, then it would be unlikely for them to develop the way they did. Each of the Mane Six, despite being horses with human minds, are emotionally independent. Rarity, for example, is a business owner. A horse aged 20 in human years is four in horse years. In those four years, could Rarity have developed such experience and maturity? As with the physical aspects, there is no indication of ponies maturing faster than humans on a mental level.

Furthermore, we can take the development of Ponyville into consideration. Because Granny Smith is known to have played a role in Ponyville's founding, we can draw conclusions based on the time it would take to develop a town of such size.

Assuming that there were no significant economic upturns in Ponyville's history, a city like this (given Equestria's fairly low population density) would take decades to build up. This is further evidenced by a lack of construction in the current time period; the town is growing slowly. It is very unlikely that Ponyville has only existed for 20-30 years. Any longer, and a horse-aging Granny Smith would have been over 100 years old in human years. It's more reasonable to assume that the town grew in the space of 50-60 years.

Within the show, everything goes to suggest that ponies are essentially humans with oddly shaped bodies. For this reason, I'm inclined to believe that the natural lifespan of an Equestrian pony is around 80 years.

On the other hand, Lauren Faust did discuss the ages of the Mane Six, shedding some light on the pony lifespan:

I personally find this statement contradictory to evidence presented in the show. Make of it what you will.